1. Field
Several aspects relate generally to wireless communications. For example, certain aspects relate more particularly to correcting error messages sent from a receiver to a transmitter.
2. Relevant Background
In a hybrid automatic repeat request (H-ARQ) system, a transmitter encodes a packet into several codewords. Each of these codewords is then sent to the receiver, as needed, in a different transmission. The receiver, thus, sees a different code rate after each transmission, with a high code rate at the end of the first transmission and a low code rate at the end of the last transmission. Such a system can be used to compensate for channel variations, channel estimation errors, etc. For example, a good channel can support a high code rate and the number of transmission required is small. By the same logic, the required number of transmissions is large in a bad channel.
The transmitter in a hybrid ARQ system utilizes feedback from the receiver to decide when to terminate transmission of the codewords. The feedback is typically in the form of an acknowledgement (ACK) or negative acknowledgment (NAK) message with an ACK indicating that the packet has been successfully decoded from the codeword(s) and a NAK indicating otherwise. These messages are often single-bit messages and, hence, are especially prone to error due to lack of channel coding gain. In other words, an ACK message can relatively easily be mistaken for a NAK and vice versa.
It has been found that an ACK→NAK error, one mistaking an ACK for a NAK, can be detected efficiently and dealt with in a manner that does not significantly affect system performance. However, a NAK→ACK error, one mistaking a NAK for a ACK, is more expensive: the transmitter believes that the packet has been successfully decoded from the codeword(s) when in fact the packet has not been successfully obtained. This causes a packet error and, thus, NAK→ACK errors impose an error floor on the overall packet error rate. These packet errors can be expensive in terms of latency since higher layer protocols, such as the Radio Link Protocol (RLP), are required to correct them. For this reason, such errors need to be detected and corrected as efficiently as possible.